1. Field of the Invention
The invention is related generally to the field of electronic and optoelectronic devices, and more particularly, to a low droop light emitting diode (LED) structure on gallium nitride (GaN) semipolar (e.g., {20-1-1}) substrates.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [X]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
At present, LEDs are utilized in high-brightness and high-power applications, such as illumination and automotive lighting, due to recent improvements in the external quantum efficiency (EQE). For commercialization of general lighting, LEDs must still overcome a major roadblock, known as “efficiency droop”.
Efficiency droop refers to the decay of the EQE at high driving current. As a result of droop, either more LED chips, or larger-area LED chips, are needed for high power devices, which results in increased wafer area and leads to higher costs.
For example, car lamp assembly and illumination manufacturers cannot completely shift to using LED devices because of their high cost. If the droop problem is resolved, these manufacturers can shift to LED devices completely, and the LED market will spread greatly.
The present invention comprises a low-droop LED structure on a GaN semipolar {20-2-1} substrate that exhibits a low Droop ratio and thus will help expedite wide spread adoption of LED lighting.
The state of the art prior to this invention includes (Al,Ga,In)N optical devices grown on polar {0001}, nonpolar {11-20} and {10-10}, and semipolar {10-1-1}, {11-22} and {20-21} GaN crystal planes. The Droop ratio of prior art devices grown on these planes is about 20% (at an emission wavelength of 430 nanometers (nm) and a drive current density of 35 Amps per centimeter square (A/cm2)), as shown in FIG. 1.
FIG. 1 is a graph of EQE (percent, %) and Light Output Power (LOP) in milliwatts (mW) vs. Current Density (A/cm2), for a prior art LED on a {20-2-1} GaN semipolar substrate. FIG. 1 shows the results for EQE for Direct Current (DC) operation (EQE (%)_DC), EQE for pulsed operation (EQE (%)_Pulse), Light Output Power for DC operation (LOP (mW)_DC), and LOP for pulsed operation (LOP (mW)_Pulse).
Thus, there is a need in the art for improved methods for reducing droop in LEDs. The present invention satisfies this need. Specifically, the present invention describes a low droop LED structure on a GaN semipolar {20-1-1} substrate.